package io.renren.common.utils;

import java.util.Random;

/**
 * twitter的snowflake算法 -- java实现
 * 1. 增加多台机器实现.
 *
 * @date 2016 /11/26
 */
public class SnowFlake {


    private final static Random RANDOM = new Random();
    /**
     * 每一部分占用的位数
     */
    private final static long SEQUENCE_BIT = 12;

    /**
     * 机器标识占用的位数
     */
    private final static long MACHINE_BIT = 5;

    /**
     * 数据中心占用的位数
     */
    private final static long DATA_CENTER_BIT = 5;

    private final static SnowFlake SNOW_FLAKE = new SnowFlake();
    /**
     * 起始的时间戳
     */
    private final static long START_STAMP = 1288834974657L;

    /**
     * 每一部分的最大值
     */
    private final static long MAX_DATA_CENTER_NUM = ~(-1L << DATA_CENTER_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);

    /**
     * 每一部分向左的位移
     */
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATA_CENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATA_CENTER_LEFT + DATA_CENTER_BIT;

    private long dataCenterId;
    private long machineId;
    private long sequence = 0L;
    private long lastStamp = -1L;

    /**
     * Instantiates a new Snow flake.
     *
     * @param dataCenterId the dataCenterId id
     * @param machineId    the machine id
     */
    private SnowFlake(long dataCenterId, long machineId) {
        if (dataCenterId > MAX_DATA_CENTER_NUM || dataCenterId < 0) {
            throw new IllegalArgumentException("dataCenterId can't be greater than MAX_DATA_CENTER_NUM or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("machineId can't be greater than MAX_MACHINE_NUM or less than 0");
        }
        this.dataCenterId = dataCenterId;
        this.machineId = machineId;
    }

    private SnowFlake() {
        this(RANDOM.nextInt((int) MAX_DATA_CENTER_NUM), RANDOM.nextInt((int) MAX_MACHINE_NUM));
    }

    /**
     * 产生下一个ID
     *
     * @return long long
     */
    private synchronized long nextId() {
        long currStamp = getNewStamp();
        if (currStamp < lastStamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (currStamp == lastStamp) {
            //相同毫秒内，序列号自增
            sequence = (sequence + 1) & MAX_SEQUENCE;
            //同一毫秒的序列数已经达到最大
            if (sequence == 0L) {
                currStamp = getNextMill();
            }
        } else {
            //不同毫秒内，序列号置为0
            sequence = 0L;
        }

        lastStamp = currStamp;

        //时间戳部分
        //数据中心部分
        //机器标识部分
        //序列号部分
        return (currStamp - START_STAMP) << TIMESTAMP_LEFT
                | dataCenterId << DATA_CENTER_LEFT
                | machineId << MACHINE_LEFT
                | sequence;
    }

    /**
     * Next id string string.
     *
     * @return the string
     */
    public String nextIdString() {
        return String.valueOf(this.nextId());
    }

    private long getNextMill() {
        long mill = getNewStamp();
        while (mill <= lastStamp) {
            mill = getNewStamp();
        }
        return mill;
    }

    private long getNewStamp() {
        return System.currentTimeMillis();
    }

    /**
     * Create snow flake snow flake.
     *
     * @return the snow flake
     */
    public static SnowFlake createSnowFlake() {
        return SNOW_FLAKE;
    }
}